This invention relates to power controls for polyphase systems in general and more particularly relates to polyphase power controls which intermittently apply groups of phase voltages from the polyphase source to the polyphase load.
Many systems for varying the operating power applied to a polyphase load in a polyphase system, such as in a polyphase induction motor system, are known, including changing the voltage applied to the load, changing both the voltage and frequency applied to the load, etc. The prior art systems typically result in unstable operation of the load and/or typically require complicated circuitry and are therefore relatively expensive, or have other disadvantages. As examples, when the speed of a polyphase induction motor is controlled by reducing the applied voltage, the slip is increased and the torque is reduced resulting in unstable operation; and changing both the voltage and the frequency of the power applied to a polyphase motor generally requires expensive circuitry which must correlate phases of the frequency to voltage amplitude.